This invention relates to a novel process of preparing chemically bonded composite hydroxide ceramics by exposing a thermally treated hydroxide ceramic to a phosphate reagent to produce a system and subsequently heat treating the system to initiate a rapid chemical bonding reaction.
Chemical reactivity in systems containing phosphoric acid or various forms of phosphates have received attention in scientific and patent literature. Particularly, the refractory applications and dental cements applications of chemical bonding (CB) of ceramics through phosphating have been disclosed. (See D. Kingery, xe2x80x9cFundamental Study of Phosphate Bonding in Refractories, Part I,II,IIIxe2x80x9d, J. Am. Cer. Soc. 33 (1950) 239-50; J. Cassidy, xe2x80x9cPhosphate Bonding Then and Nowxe2x80x9d, Am. Cer. Soc. Bull. 56 (1977)640-43; J. Bothe and P. Brown, xe2x80x9cLow Temperature Formation of Aluminum Orthophosphatexe2x80x9d, J. Am. Cer. Soc. 76 (1993) 362-68; and J. Bothe and P. Brown, xe2x80x9cReactivity of Alumina towards Phosphoric Acidxe2x80x9d, J. Am. Cer. Soc. 76 (1993) 2553-58.) For example, mixing aluminium oxide, or alumino-silicates, or zircon, or many other pure or mixed oxides (such as Cr2O3, ZrO2with phosphoric acid H3PO4 (PA) or monoaluminum phosphate Al(H2PO4)3, (MAP) leads to reaction between the constituents and formation of chemical bond at relatively low temperatures of 200-400xc2x0 C. These processes yield successful, commercial refractory materials. (See D. Kingery, xe2x80x9cFundamental Study of Phosphate Bonding in Refractories, Part I,II,IIIxe2x80x9d, J. Am. Cer. Soc. 33 (1950) 239-50.) The objective of these inventions was to produce monolithic ceramic while avoiding the usual high-temperature treatment (or xe2x80x9csinteringxe2x80x9d) necessary to bond ceramic particles. Additionally, chemically bonded ceramics experience very small shrinkage processing, i.e. size and shape of the resulting chemically bonded component is approximately the same as those of the mixed and pressed powder component.
In another example of known prior art, zinc oxide mixed with zinc metal and aluminium hydroxide is further mixed with phosphoric acid. The mixture reacts and sets at room temperature yielding dental cement. The cementitious behaviour of phosphate-containing systems has been explored on a large scale if one of the oxides in the system exhibits substantial room temperature reactivity towards phosphates. For example, MgO rapidly reacts with monoaluminum phosphate to form hydrated magnesium phosphates that bond the aggregate components of the cold-setting concrete. Reaction bonding of alumina with phosphates has also been used to produce ceramics of controlled, fine pore structure such as molecular sieves U.S. Pat. No. 5,178,846. Phosphating of steel or aluminium produces a thin (1-10 xcexcm) mildly protective layer which can be utilised as a bondcoat for subsequent application of organic paints, or other coatings, such as ceramic coatings.
In order to produce a phosphate-bonded ceramic, a chemical reaction is initiated between the phosphate-carrying reactant, for example orthophosphoric acid (H3PO4), and an oxide (such as alumina, zirconia, chromia, zinc oxide, and others). As a result, refractory phosphates, such as aluminium phosphate, are formed at relatively low temperatures. For example, for the system Al2O3xe2x80x94H3PO4xe2x80x94Al(H2PO4)3, the reaction starts at 127xc2x0 C., and is complete at about 500xc2x0 C. At higher temperatures, the resulting amorphous aluminophosphates undergo a chain of crystallization-phase transformations, to eventually decompose to P2O5 and Al2O3 above 1760xc2x0 C. (See Bothe and P. Brown, xe2x80x9cLow Temperature Formation of Aluminum Orthophosphatexe2x80x9d, J. Am. Cer. Soc. 76 (1993) 362-68; and J. Bothe and P. Brown, xe2x80x9cReactivity of Alumina towards Phosphoric Acidxe2x80x9d, J. Am. Cer. Soc. 76 (1993) 2553-58.)
The systems of particular interest include ceramic particles that are chemically bonded to form a protective film on metallic substrate. The films can be used for surface modification in preparation for deposition of subsequent coatings (e.g. phosphate treatment of metals before painting) or for added protection against corrosion and/or wear. However, due to substantial reactivity of phosphates, e.g. phosphoric acid, towards metals, the systems involving metals (e.g. phosphate-containing coatings on metals, or coatings that contain metallic particles) must include means of controlling reactivity of such system. One of such systems disclosed in the scientific and patent literature, is a protective coating for metals (e.g. steel) that contains simultaneously phosphoric acid and aluminium metal particles. In such coating particulate aluminium is combined with the phosphoric acid solution, applied to surface of metal, and heat treated at 250-550xc2x0 C. to bond the metal particles together, and to the substrate base metal. In such a coating formulation aluminium must be protected from extensive, and possibly violent, reaction with the phosphate. One of the best-known systems that achieve this objective has been disclosed in U.S. Pat. No. 3,248,251, where chromates or molybdates were added to the solution to effectively protect aluminium metal from excessive reaction with the phosphate. These predominantly metallic coatings are still widely applied to protect ferrous metals form corrosion and oxidation. Another similar system has been disclosed in U.S. Pat. No. 3,395,027. In an attempt to eliminate the use of environmentally dangerous chromates or molybdates, formulations rich in dissolved aluminium ions, e.g. less reactive towards aluminium metals, have been proposed by Stetson et al in U.S. Pat. Nos. 5,279,649 and 5,279,650. These formulations contained numerous other substances that were supposed to inhibit reactivity of phosphates towards aluminium particles. Yet another attempt to produce xe2x80x9cenvironmentally friendlyxe2x80x9d phosphate bonding composition suitable for coatings is disclosed by Mosser et al in a series of U.S. Pat. Nos. 5,478,413; 5,652,064; 5,803,990 and 5,968,240. All of these formulations include complex mixtures of ions (in addition to the phosphate ion solution) with the objective to control reactivity of phosphates in coatings application. In one variant of such coating system, disclosed in U.S. Pat. No. 4,544,408, a water/acid dispersion premix of hydrated alumina (e.g. boehmite or pseudoboehmite) is admixed into the usual chromate/phosphate or molybdate/phosphate coating composition. The patent teaches that mixing the two solutions leads to gelation of the hydrated alumina particles and, as a result of this process, a thixotropic mixture is formed. The thixotropic nature of the mixture allows deposition of uniform coatings in the spin coating process. It is disclosed that particles of alumina or aluminium improve performance of such coatings. It is further claimed in U.S. Pat. No. 4,838,942 that the coating system containing aluminium particles and a mix of chromic, phosphorous, phosphoric acids and aluminium phosphate can be cured at very low temperature of 150xc2x0 C. to 190xc2x0 C.
Another area pertaining to the present invention includes fully ceramic systems (i.e. no metals are present) where very fine particles (nanometer size) of hydroxide ceramic (HC), such as boehmite AlOOH, are mixed with calcined ceramic, such as alpha aluminium oxide. (See S. Kwon and G. L. Messing, xe2x80x9cSintering of Mixtures of Seeded Bohemite and Ultrafine Alpha Aluminaxe2x80x9d, J. Am. Cer. Soc. 83 (2000) 82-88; and M. Kumagai and G. L. Messing, xe2x80x9cControlled Transformation and Sintering of a Bohemite Sol-Gel by Alpha Alumina Seedingxe2x80x9d, J. Am. Cer. Soc. 68 (1985) 500-505.) These systems are referred to in the present invention as composite hydroxide ceramic CHC. During heat treatment the nanometer-size particles of HC decompose, releasing water, and form very active nanometer-size particles of aluminium oxide. In these systems the very large surface area (in excess of 100 m2/gram), and thus high reactivity, of the thermally decomposed boehmite is utilised to accelerate sintering of the resulting aluminium oxide, i.e. full densification of such CHC is achieved at about 1300xc2x0 C. The initially admixed particles of calcined alpha aluminium oxide act as nucleation site for alumina forming from thermally decomposing boehmite. These systems are useful in processing of dense alumina ceramics at relatively low temperatures, i.e. 1300xc2x0 C. However, these temperatures are too high to be able to process ceramic coatings of CHC on most metals. Moreover, thermal decomposition of boehmite and consequent removal of water from such system leads to relative large shrinkage, in excess of 20%, of the resulting ceramic body. Thus the hypothetical CHC coating would crack during heat treatment, as the base metal would not experience any processing shrinkage. This shrinkage decreases if share of the calcined ceramic in the system increases. This phenomenon is well known in the prior art of ceramic processing, e.g. in refractory ceramic processing. For example, when compacting a refractory brick of ceramic powders that could substantially shrink upon heat treatment (e.g. clay components), a portion of previously fired and ground brick (xe2x80x9cgrogxe2x80x9d) is added to decrease the overall shrinkage, while maintaining chemical composition of the resulting brick essentially unchanged.
The same concept, as applied to ceramic coatings, has been introduced by Barrow et al. (xe2x80x9cThick Ceramic Coatings using a Sol Gel Based Ceramicxe2x80x94Ceramic 0-3 Composite:, Surf. Coat. Tech., 76-77 (1995) 113) and disclosed in U.S. Pat. No. 5,585,136, re-issued as Re. No. 36,573. The authors teach that dispersion of up to 90% of fine calcined ceramic particles into sol-gel solutions allows depositing ceramic coatings and thick composite films on metals. The disclosed sol-gel solutions are obtained through relatively complex route of dissolving salts, organometalic compounds, such as alkoxides, or carboxylates and ketones. These systems, which can be classified as composite sol-gel (CSG), still need to be heat treated at relatively high temperature up to about 1000xc2x0 C., to initiate ceramic bond formation.
A method to avoid these excessive temperatures and still achieve substantial ceramic bond in CSG has been recently disclosed by Troczynski and Yang, U.S. Pat. No. 6,284,682 B1, granted Sep. 4, 2001. In that invention, CSG ceramic coatings are subject to chemical bonding through phosphating reactions, i.e. by impregnation of CSG coatings with phosphoric acid H3PO4 (PA) or monoaluminum phosphate Al(H3PO4)3, (MAP), or combination thereof. The high reactivity of the sol particles produced through dissolving salts, organometalic compounds, such as alkoxides, or carboxylates and ketones, allows rapid chemical bonding of CSG coatings at temperatures as low as 200xc2x0 C. The chemically bonded composite sol-gel system is obtained (CBxe2x80x94CSG), useful for deposition of fully ceramic coatings on metals at low temperatures. Phosphating of aluminium salts has been used in the past (U.S. Pat. No. 4,927,673) for rapid hardening of molds for casting metals.
The present invention represents an inventive advance over the phosphate containing systems described above.
An objective of the present invention is to control reactivity of phosphates, such as phosphoric acid H3PO4 (PA), phosphorous acid H3PO3 (PAxe2x80x2), or monoaluminum phosphate Al(H3PO4)3, (MAP), towards other components of the system (metallic or non-metallic), including molded ceramics and ceramic coatings, without any addition of secondary compounds such as molybdates or chromates. Such well-controlled phosphate systems allow, for example, deposition of environmentally clean ceramic coatings on metals or non-metals.
A further objective of this invention is to utilize the very high activity of thermally decomposed fine hydroxide ceramic, such as thermally dehydrated boehmite ceramic, to initiate chemical bonding through phosphating, and to control reactivity of the phosphates, such as phosphoric acid H3PO4 (PA), phosphorous acid H3PO3 (PAxe2x80x2), or monoaluminum phosphate Al(H3PO4)3, (MAP), towards other components of the system (metallic and/or non-metallic).
A further objective of this invention is to use simple composite hydroxide ceramic (CHC) systems, such as boehmite ceramic mixed with calcined ceramic (for example alumina powder), to minimise shrinkage of the resulting ceramic coating, such that crack-free coatings are obtained.
A key feature of the subject invention is its ability to achieve high quality, dense ceramic coatings on metals and non-metals at low process temperatures, while avoiding complex ionic content, or complex sol-gel routes through dissolving salts, organometalic compounds, such as alkoxides, or carboxylates and ketones. The subject invention therefore discloses that suitable thermal treatment of the composite hydroxide ceramic (CHC), followed by chemical bonding (CB) through phosphating of CHC, can yield excellent, environmentally friendly chemically bonded composite hydroxide ceramics (CBxe2x80x94CHC). These materials are especially suitable as ceramic coatings.
One example of a procedure according to the invention is a mix of calcined alumina and hydroxide alumina (or hydrated alumina, such as boehmite) networks, which is heat treated at about 200xc2x0 C. to de-hydrate the hydroxide and is then impregnated with a mix of metal phosphate and phosphorus acid. The phosphates and phosphorus acid react primarily with the active hydroxide derived alumina networks to form complex amorphous phosphates at about 300xc2x0 C., which can crystallize upon heat treatment above about 600xc2x0 C. The hydrated alumina derived alumina is subject to reaction with phosphoric acid to result in a polymerized network of monoaluminum phosphate. The phosphates also partially react with calcined alumina, thereby providing a strong bond between the alumina filler particles, and the continuous matrix phosphate phase. In order to achieve the desired properties, the kinetics of the phosphating reaction must be controlled to prevent substantial reaction of the phosphate with the substrate, which is undesirable because it can lead to reaction product buildup at the interface and spallation of the coating. The kinetic reaction control is achieved through the use of an active hydroxide-derived phase (e.g. through dehydration of boehmite) with an inert ceramic filler, and phosphate phase, in proper ratio, particle size, and concentration across the coating.
The invention is directed to a process of preparing a chemically bonded ceramic comprising phosphating a hydroxide derived oxide or hydrated oxide ceramic with heat treatment at a temperature between about 200xc2x0 C. and about 1200xc2x0 C.
The hydroxide derived oxide can be a first phase and can be impregnated with a secondary phosphate phase which can react with the oxide ceramic first phase. In one embodiment of the invention, a mixture of calcined alumina and hydroxide alumina, such as boehmite (AlOOH), can be heat treated in air at about 200xc2x0 C. to 300xc2x0 C. to decompose the hydroxide alumina, and then impregnated with a mixture of metal phosphate and phosphorus acid to form complex amorphous phosphates which can crystallize under further heat treatment. In another embodiment of the invention, porosity in the surface of the ceramic coating can be sealed by utilizing a process selected from the group consisting of hydroxide impregnation, hydroxide electrophoretic deposition, aluminum phosphate impregnation, phosphorus acid impregnation, or a combination of these treatments.
In a specific embodiment, the invention involves a process of preparing a chemically bonded ceramic comprising: (a) as a first step, preparing a slurry of solvent and hydroxide ceramic: (b) as a second step, heat treating the hydroxide ceramic slurry at a temperature of between about 100 to 800xc2x0 C. to produce a dehydrated oxide ceramic: (c) as a third step, impregnating the dehydrated oxide ceramic with a phosphating agent; and (d) as a forth step, heat treating the phosphate impregnated oxide ceramic at a temperature between 200xc2x0 C. and 1200xc2x0 C. to seal pores in the ceramic and produce a phosphated oxide ceramic powder to produce a mixed slurry; (b) applying said mixed slurry to a substrate to thereby coat the substrate with the ceramic hydroxide slurry; (c) heating the ceramic hydroxide coated substrate at a temperature up to about 300xc2x0 C. to 1000xc2x0 C. to produce a ceramic metal oxide film on the substrate; and (d) sealing surface pores of the ceramic coating with a phosphorus containing ceramic sealant.
The hydrated ceramic oxide can be one or more of SiO2, Al2O3, ZrO2, TiO2, BeO, SrO, BaO, CoO, NiO, ZnO, PbO, CaO, MgO, CeO2, Cr2O3, Fe2O3, Y2O3, Sc2O3, HfO2 or La2O3. The phosphating agent can be a metal phosphate, phosphoric acid, or mixtures thereof. The metal in the phosphate can be one or more of Al, Zr, Ti, Mg, Cu, Fe, Ca, Sr, Hf or Cr. The process can include a calcined ceramic filler comprised of powders or fibers of oxides, carbides, nitrides, borides, fluorides, suphides or mixtures thereof.
The invention is also directed to a process of preparing a chemically bonded hydroxide ceramic coating deposited on a substrate comprising phosphating a hydroxide derived oxide ceramic deposited on the substrate for sufficient time to seal pores in the ceramic, but for insufficient time to attack the substrate, and polymerizing the resulting product with heat treatment at a temperature between about 200xc2x0 C. and about 1200xc2x0 C.
The invention also pertains to a method of preparing a ceramic hydroxide coating on a substrate which comprises: (a) immersing a substrate coated with a ceramic hydroxide coating into a solution containing dispersed boehmite; (b) withdrawing the ceramic hydroxide coated substrate from the boehmite alumina solution and drying the coated substrate at a temperature of about 100xc2x0 C.; (c) heat treating the dried coated substrate at a temperature of about 200xc2x0 C. for about 10 min. to substantially dehydrate the boehmite; (d) applying a phosphoric acid solution on the surface of the ceramic coated substrate to seal the pores in the ceramic coating for sufficient time that permits the alumina resulting from dehydration of boehmite to react with the phosphoric acid solution sufficiently rapidly to seal pores in the alumina ceramic coating, but for insufficient time that the underlying substrate is exposed to substantial phosphating reaction. The phosphoric acid can be reacted with the dehydrated boehmite coating at about 300xc2x0 C. for about 10 minutes. The reaction of the phosphoric acid with the dehydrated boehmite ceramic coating can yield a polymerized network of mono-aluminum phosphate within and on the surface of the coating.
The invention is also directed to a method of preparing an alumina/alumina composite hydroxide ceramic (CHC) coating on a substrate which comprises mixing boehmite and calcined aluminium oxide in water at a pH of 2 to 6 to produce a suspension, agitating the suspension to produce a homogenous slurry, immersing a Substrate in the slurry to coat the substrate, and drying the composite hydroxide ceramic coating at a temperature of 50 to 200xc2x0 C. Other ceramic particles, such as zirconia particles can be substituted for the calcined aluminum oxide. The porous coating resulting from this process is then sealed using chemical bonding process through phosphating, as described above. Alternatively, the solvent can be methyl alcohol, ethyl alcohol or isopropyl alcohol.
The invention also pertains to a method of sealing porosity of ceramic coatings comprising impregnating the porous ceramic coating with mono-aluminum phosphate for about 10 to 50 min., and heating the impregnated coatings at a temperature of about 300xc2x0 C. for about 20 to 50 min. The product may be further heat treated at temperatures of 500 to 800xc2x0 C. for about 10 to 50 min to crystallize the polymerized complex glassy phosphases resulting from the reaction.
The invention also includes a method of sealing a composite hydroxide ceramic coating on a substrate with phosphoric acid which comprises treating the coating with phosphoric acid for about 1 to 20 min., and polymerising the resulting product at a temperature of about 300xc2x0 C. for 20 to 50 min.
The invention is also directed to a process for producing a porosity sealed ceramic film on a substrate comprising: (a) mixing a hydroxide solution with a metal oxide ceramic powder to produce a mixed slurry; (b) applying said mixed slurry to a substrate to thereby coat the substrate with the ceramic hydroxide slurry; (c) heating the ceramic hydroxide coated substrate at a temperature up to about 600xc2x0 C. to 1000xc2x0 C. to produce a ceramic metal oxide film on the substrate; and (d) sealing surface pores of the ceramic coating with a phosphorus containing ceramic sealant.
The ceramic sealing process can be selected from the group of processes comprising hydroxide impregnation, hydroxide electrophoretic deposition, aluminum phosphate impregnation or phosphorus acid impregnation.